Voltage detection apparatus

ABSTRACT

This invention has as its object to provide a voltage measurement apparatus which has a compact probe unit, and which can perform a measurement in a non-contact manner. The voltage measurement apparatus includes detection means for detecting an electric field generated in a space by a voltage applied to the surface of a device to be measured, light-emitting means for modulating output light by superposing a detected signal obtained from the detection means on a bias current which is supplied to inductively radiate the output light, a constant current source for supplying the bias current to the light-emitting means, extraction means for extracting a signal component of the output light from the light-emitting means, and light-transmission means for guiding the output light from the light-emitting means to the extraction means, and measures the applied voltage to the surface of the device to be measured by bringing the detection means close to the device to be measured in a non-contact manner.

This is a continuation of application Ser. No. 08/186,580, filed on Jan.26, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a voltage measurement device fordetecting a very low voltage applied to a local portion of a device tobe measured.

2. Related Background Art

As conventional methods for measuring a very low voltage signal appliedto a local portion of a device to be measured, the first method formeasuring a voltage signal by contacting a probe of an oscilloscope toan electrical circuit to be measured, the second method for opticallymeasuring a voltage signal using an E-O probe consisting of anelectro-optic crystal, and the like are known. As an example of thesecond measurement method, a method disclosed in Japanese PatentLaid-Open No. 156379/1991 is known. This reference discloses a techniquefor detecting an electrical signal applied to an electrical circuit tobe measured without bringing the E-O probe into contact with a device tobe measured.

However, in the first measurement method, since a measurement isperformed by bringing the probe into direct contact with an electricalcircuit to be measured, the constant of the electrical circuitundesirably changes. Furthermore, a signal waveform upon transmission isdistorted due to the distributed constant of a transmission line forconnecting the probe and a main body, or external noise is superposed onan electrical signal on the transmission line, thus deteriorating theS/N ratio. Owing to these problems, it is difficult to measure a trueelectrical signal by the measurement method using the oscilloscope.

It is difficult to measure a plurality of points at the same time by thesecond measurement method. Furthermore, due to a very smallelectro-optic crystal, and a large number of optical elements, it isdifficult to align the optical elements.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned problems, a voltage detectionapparatus according to the first aspect of this comprises: detectionmeans for detecting the strength of an electric field generated in aspace by a voltage applied to the surface of a device to be measured;light-emitting means for modulating output light with a signal detectedby the detection means; and extraction means for extracting a signalcomponent of the output light from the light-emitting means.

The detection means may comprise a metal electrode having a rod shapewith a sharp distal end, or a dipole shape, or may comprise aneedle-shaped metal electrode inserted in a hollow tube consisting of anon-conductive material. Furthermore, light-transmission means forguiding the output light from the light-emitting means to the extractionmeans may be arranged.

Since the voltage detection apparatus according to the first aspect ofthis has the above-mentioned arrangement, when the detection means isbrought close to the surface of a device to be measured, an electricfield generated in a space by a voltage applied to the surface of thedevice to be measured is detected by the detection means as a detectedsignal corresponding to the electric field strength. The detected signalis supplied to the light-emitting means to modulate output light. When asignal component of the modulated output light is extracted by theextraction means, the voltage applied to the surface of the device to bemeasured can be detected.

In this case, when the detection means comprises a rod-shaped metalelectrode with a sharp distal end or a needle-shaped metal electrodeinserted in a hollow tube, a voltage signal on a very small circuit suchas an IC circuit, which is difficult to observe by human eyes, can beeasily detected. When the detection means comprises a dipole-shapedmetal electrode, a voltage signal on an elongated device to be measuredsuch as a metal wiring pattern can be detected with high sensitivity.

In this manner, since the applied voltage is detected without contactingthe detection means with the device surface, the applied voltage on thesurface of the device to be measured can be prevented from beingdistorted, or superposition of noise can be prevented. For this reason,a voltage applied to the surface of the device to be measured can beprecisely detected.

Since the detection means and the light-emitting means can be easilyintegrated, the apparatus can be rendered compact. Furthermore, sincethe light-emitting means outputs an optical signal, and simultaneouslymodulates the optical signal, the number of optical elements can bedecreased as compared to a conventional voltage detection apparatuswhich performs outputting and modulation of an optical signal usingdifferent optical elements. For this reason, alignment between, e.g.,the optical axes of optical elements is facilitated, and the apparatuscan be stabilized.

When the light-transmission means is arranged between the light-emittingmeans and the extraction means, a probe unit consisting of the detectionmeans and the light-emitting means, and the extraction means can beseparated away from each other, and waveform distortion or mixing ofexternal noise during transmission can be suppressed as much aspossible. If the light-transmission means comprises an optical fiber,the probe unit and the extraction means can be freely arranged.

A voltage detection apparatus according to the second aspect of thiscomprises: a semiconductor laser for detecting the strength of anelectric field generated in a space by a voltage applied to the surfaceof a device to be measured by bringing a lower-surface electrode closeto the surface of the device to be measured, and modulating output lightwith the detected signal; a constant current source for supplying a biascurrent to the semiconductor laser; extraction means for extracting asignal component of the output light from the semiconductor laser; andlight-transmission means for guiding the output light from thesemiconductor laser to the extraction means.

Since the voltage detection apparatus according to the second aspect ofthis has the above-mentioned arrangement, when the lower-surfaceelectrode of the semiconductor laser is brought close to the surface ofa device to be measured, an electric field generated in a space by avoltage applied to the surface of the device to be measured is detectedby the lower-surface electrode as a detected signal corresponding to theelectric field strength. This detected signal is superposed on the biascurrent supplied from the constant current source to the semiconductorlaser. Upon superposition of the detected signal, output lightinductively radiated from the semiconductor laser is modulated. Whenlight is inductively radiated from the semiconductor laser, the outputlight intensity is proportional to the supplied current, and the ratiobetween the light and the current is constant. For this reason, in thissemiconductor laser, the output light intensity increases/decreases incorrespondence with the detected signal supplied upon being superposedon the bias current.

When a signal component of the modulated output light is extracted bythe extraction means, a voltage applied to the surface of the device tobe measured can be detected.

In this manner, since the applied voltage is detected without contactingthe lower-surface electrode of the semiconductor laser, the appliedvoltage on the surface of the device to be measured can be preventedfrom being distorted, or superposition of noise can be prevented. Forthis reason, a voltage applied to the surface of the device to bemeasured can be precisely detected.

Furthermore, since the semiconductor laser outputs an optical signal,and simultaneously modulates the optical signal, the number of opticalelements can be decreased as compared to a conventional voltagedetection apparatus which performs outputting and modulation of anoptical signal using different optical elements. For this reason,alignment between, e.g., the optical axes of optical elements isfacilitated, and the apparatus can be stabilized.

When the light-transmission means is arranged between the semiconductorlaser and the extraction means, the semiconductor laser and theextraction means can be separated away from each other, and waveformdistortion or mixing of external noise during transmission can besuppressed as much as possible. If the light-transmission meanscomprises an optical fiber, the semiconductor laser and the extractionmeans can be freely arranged.

Furthermore, a voltage detection apparatus according to the third aspectof this comprises: a plurality of two-dimensionally arranged detectionelectrodes each for detecting the strength of an electric fieldgenerated in a space by a voltage applied to the surface of a device tobe measured; a semiconductor laser array, consisting of a plurality ofsemiconductor lasers corresponding to the detection electrodes, formodulating output light beams by superposing detected signals from theplurality of detection electrodes on a bias current which is supplied toinductively radiate output light beams; a constant current source forsupplying a bias current to the semiconductor laser array; extractionmeans for extracting signal components of the output light beams fromthe semiconductor lasers of the semiconductor laser array; andlight-transmission means for guiding the output light beams from thesemiconductor laser array to the extraction means.

Since the voltage detection apparatus according to the third aspect ofthis has the above-mentioned arrangement, when the plurality oftwo-dimensionally arranged detection electrodes are brought close to thesurface of a device to be measured, an electric field generated in aspace by a voltage applied to the surface of the device to be measuredis detected by the detection electrodes as detected signalscorresponding to the electric field strength. These detected signals aresupplied to the corresponding semiconductor lasers of the semiconductorlaser array, and are superposed on the bias current supplied from theconstant current source to the semiconductor lasers. Upon superpositionof the detected signals, output light beams to be inductively radiatedfrom each semiconductor laser are modulated.

When the signal components of the modulated output light beams areextracted by the extraction means, a voltage signal on the device to bemeasured can be two-dimensionally detected, and the distribution of avoltage applied to the surface of the device to be measured can bedetected.

When the light-transmission means is arranged between the semiconductorlaser array and the extraction means, probe units consisting of theplurality of detection electrodes and the semiconductor laser array, andthe extraction means can be separated away from each other, and waveformdistortion or mixing of external noise during transmission can besuppressed as much as possible. If the light-transmission meanscomprises an optical fiber array, the probe units and the extractionmeans can be freely arranged.

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not to beconsidered as limiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the structure of a voltagedetection apparatus according to an embodiment of the present invention;

FIG. 2 is a perspective view showing another structure of the voltagedetection apparatus according to the embodiment of the presentinvention;

FIG. 3 is a plan view showing an arrangement of a signal processor;

FIG. 4 is a plan view showing another arrangement of the signalprocessor;

FIG. 5 is a circuit diagram showing the relationship between the appliedvoltage and the output light intensity of a semiconductor laser;

FIG. 6 is a graph showing the relationship between the output lightintensity and the injection current;

FIG. 7 is a graph showing the relationship between the applied voltageand the injection current;

FIG. 8 is a circuit diagram of a voltage detection circuit by modulatinga semiconductor laser by superposing an input signal voltage;

FIG. 9 is a circuit diagram of a voltage detection circuit provided withan electrode for measuring an input signal voltage;

FIG. 10 is a circuit diagram of a voltage detection circuit when anelectrode for measuring an input signal voltage is brought into contactwith a device to be measured in a non-contact manner;

FIG. 11 is a circuit diagram of a voltage detection circuit when anelectrode for measuring an input signal voltage comprises parallelplates;

FIG. 12 is a graph showing the distribution of actually measured data ofa modulated output with respect to the voltage amplitude of anelectrical signal;

FIG. 13 is a perspective view showing an arrangement of a voltagedetection apparatus according to the embodiment of the presentinvention;

FIG. 14 is a perspective view showing an arrangement of a probe unitusing a surface-emission laser;

FIG. 15 is a sectional view showing the arrangement of the probe unitusing the surface-emission laser;

FIG. 16 is a perspective view showing a modification in which theelectrode is formed to have a dipole antenna shape;

FIG. 17 is a perspective view showing a modification in which theelectrode is formed to have a needle shape, and is covered with anon-conductive material such as a ceramic;

FIG. 18 is a perspective view showing a modification using asurface-emission semiconductor laser array;

FIG. 19 is a sectional view showing the modification using thesurface-emission semiconductor laser array; and

FIG. 20 is a perspective view showing an application using theembodiment of the present invention in an IC inspection apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described below withreference to the accompanying drawings. FIG. 1 is a perspective viewshowing the structure of a voltage detection apparatus according to thisembodiment. The voltage detection apparatus of this embodiment comprisesa probe unit 10 for detecting the electric field strength of a devicesurface upon being brought close to the surface of a device to bemeasured, and outputting the electric field strength as an opticalsignal, an optical fiber 20 for transmitting an optical signal outputfrom the probe unit 10, and a photodetector 30 for converting theoptical signal output from the probe unit 10 into an electrical signal.The apparatus also comprises a signal processor 40 for extracting asignal component included in the electrical signal converted by thephotodetector 30, and a constant current source 50 for supplying acurrent to the probe unit 10.

The probe unit 10 comprises an electrode 11 for detecting the strengthof an electric field generated in a space by a voltage signal on thesurface of a device to be measured, a semiconductor laser 12 formodulating output light in correspondence with the electrical signalgenerated by the electric field strength detected by the electrode 11,and a focusing lens 13 for focusing light output from the semiconductorlaser 12 at an input terminal 20a of the optical fiber 20. Thephotodetector 30 comprises a focusing lens 31 for focusing output lightfrom the optical fiber 20, and a photoelectric conversion element 32 forconverting an optical signal focused by the focusing lens 31 into anelectrical signal. The focusing lens 31 and the photoelectric conversionelement 32 are stored in an air-tight package 33.

In this embodiment, the electrode 11 adopts a rod-shaped electrode witha sharp distal end, and the semiconductor laser 12 and the focusing lens13 are packaged in an air-tight package 14. The air-tight package 14 ismanufactured using an insulating material. With this package,deterioration of the semiconductor laser 12 can be prevented, andoptical axis adjustment can be simplified, thus improving reliability.

The operation of this embodiment is as follows. Upon current supply fromthe current source 50, a bias current flows through the semiconductorlaser 12, and a laser beam is inductively radiated. The output light isfocused by the focusing lens 13, and is supplied to the input terminal20a of the optical fiber 20. The light supplied to the input terminal20a is transmitted through the interior of the optical fiber 20, emergesfrom an output terminal 20b, and is focused on the input surface of thephotoelectric conversion element 32 by the focusing lens 31.

When the distal end of the electrode 11 is brought close to a device tobe measured, a current flows through the electrode 11 by an electricfield on the surface of the device to be measured. Since this current issuperposed on a bias current supplied from the current source 50, theoutput light inductively radiated from the semiconductor laser 12 ismodulated. When the laser beam is inductively radiated from thesemiconductor laser 12, the intensity of the output light isproportional to the supplied current, and the ratio between theintensity and the current is constant. For this reason, the intensity ofthe output light from the semiconductor laser 12 increases/decreases incorrespondence with the detected signal supplied upon being superposedon the bias current. The modulated output light is focused on thephotoelectric conversion element 32, and is converted into an electricalsignal, thereby detecting a voltage applied to the surface of the deviceto be measured.

FIG. 2 is a perspective view showing a modification of this embodiment.A difference between this modification and the embodiment shown in FIG.1 is that the proximal end portion of the rod-shaped electrode 11 with asharp distal end is ground to form a flat portion, and the semiconductorlaser 12 is arranged on this flat portion to achieve a further compactstructure. Since the semiconductor laser 12 contacts the flat portion,so that its lower-surface electrode 12a is electrically connected to theelectrode 11, a transmission line for connecting the electrode 11 andthe lower-surface electrode 12a of the semiconductor laser 12 can beomitted. For this reason, a signal distortion and mixing of externalnoise during transmission can be suppressed as much as possible. Thereason why the semiconductor laser 12 can be arranged on the electrode11 is that one side of the chip of the semiconductor laser 12 is assmall as 300 μm.

In the embodiments shown in FIGS. 1 and 2, since the rod-shapedelectrode 11 with a sharp distal end is used, only an electric field ina narrow portion of the surface of the device to be measured to thevicinity of which the electrode 11 is brought for measurement iscoupled. For this reason, the coupling efficiency becomes low, but thespatial resolution can be improved. In these embodiments, the opticalfiber 20 is used as light-transmission means for guiding output lightfrom the semiconductor laser 12 to the photodetector 30. For thisreason, the probe unit 10 and the photodetector 30 can be separated awayfrom each other. Also, a problem associated with disturbance of anelectric field on the surface of the device to be measured caused by ametal transmission line can be avoided.

The arrangement of the signal processor 40 will be described below withreference to the plan views in FIGS. 3 and 4. Output light from thesemiconductor laser 12 is photoelectrically converted by thephotoelectric conversion element 32 such as a photodiode to obtain amodulated electrical signal. The modulated electrical signal is suppliedto the signal processor 40, and a signal component superposed on a DCcomponent is detected. More specifically, the modulated electricalsignal is separated by capacitance coupling using a capacitor 41. Asshown in FIG. 3, the separated signal is amplified by an amplifier 42,and is measured using an oscilloscope 43. Alternatively, as shown inFIG. 4, the signal may be measured using a lock-in amplifier 44.Furthermore, for example, if an accumulation is performed using astorage oscilloscope to increase the S/N ratio, detection precision canbe improved.

The operation principle of this embodiment will be described below withreference to FIGS. 5 to 12. FIG. 5 is a circuit diagram showing therelationship between an applied voltage V_(LD) to a semiconductor laser60, and an output light intensity P_(LD) from the semiconductor laser60. When an injection current I_(LD) is increased by adjusting a currentsource 61 in this circuit, since the semiconductor laser 60laser-oscillates when the current exceeds a threshold current I_(th),the output light intensity P_(LD) from the semiconductor laser 60immediately increases. FIG. 6 shows the relationship between the outputlight intensity P_(LD) and the injection current I_(LD). As can be seenfrom FIG. 6, a constant slope efficiency η(=ΔP_(LD) /Δ_(LD)) is obtainedafter laser oscillation. Also, FIG. 7 shows the relationship between theapplied voltage V_(LD) and the injection current I_(LD) of thesemiconductor laser 60. As can be seen from FIG. 7, even if theinjection current I_(LD) increases, the voltage V_(LD) does not changeso much. Also, as can be seen from FIG. 7, the inclination (ΔI_(LD)/ΔV_(LD)) between the injection current I_(LD) and the voltage V_(LD) isalmost constant. This is because a pn junction portion of thesemiconductor laser 60 serves as a diode. Normally, a current I_(LD) atwhich the inclination between the voltage V_(LD) and the current I_(LD)begins to become constant satisfies I_(LD) <<I_(th). Therefore, when thecurrent I_(LD) is equal to or higher than the current I_(th), thevoltage change amount ΔV_(LD) is proportional to the current changeamount ΔI_(LD). For this reason, the change amount ΔP_(LD) of the outputlight intensity and the change amount ΔV_(LD) of the applied voltagehave a proportional relationship therebetween. With this relationship,the output light can be modified by a change in applied voltage to thesemiconductor laser 60.

In this manner, by utilizing the principle of modulating the outputlight by a change in applied voltage to the semiconductor laser 60, aninput signal voltage V_(sig) of a device to be measured can be measured.More specifically, using a circuit shown in FIG. 8, the input signalvoltage V_(sig) of the device to be measured can be superposed on a biascurrent I_(b) by the capacitance coupling, and the output light from asemiconductor laser 70 is modulated by a change in voltage of the inputsignal voltage V_(sig). A more general circuit of the circuit shown inFIG. 8 is the one shown in FIG. 9. In this circuit, an electrode 75 isarranged between a capacitor 71 and an input signal source 73, and whenthis electrode 75 is brought into contact with a measurement point 76aof a circuit 76 to be measured, an input signal at the measurement point76a is superposed on the bias current I_(b). Upon superposition of thisinput signal, the output light from the semiconductor laser 70 ismodulated. When the modulated output light is measured, an electricalsignal at the measurement point 76a can be measured.

However, when the electrode 75 is brought into direct contact with themeasurement point 76a of the circuit 76 to be measured as in the circuitshown in FIG. 9, the circuit constants of the semiconductor laser 70,the electrode 75, and the like influence the circuit 76 to be measured,and the characteristic of the circuit 76 to be measured undesirablychanges. For this reason, an electrical signal of the circuit 76 to bemeasured is distorted, and is measured in a characteristic differentfrom an actual state. In order to suppress such influence on the circuit76 to be measured, and to measure a correct electrical signal, ameasurement must be performed while the electrode 75 and the circuit 76to be measured are kept in a non-contact state.

Thus, this embodiment adopts a method wherein the electrode 75 isbrought close to the circuit 76 to be measured in a non-contact state tomeasure an electrical signal of the circuit 76 to be measured. FIG. 10is a circuit diagram of this circuit. As the circuit 76 to be measured,an input signal source 73 and a resistor 77 are used. In this circuit,the measurement point 76a of the circuit 76 to be measured iscapacitively coupled to the electrode 75, and if their couplingefficiency is represented by α, a voltage change amount ΔV_(sig) at themeasurement point 76a of the circuit 76 to be measured is applied to thesemiconductor laser 70 as a voltage change amount ΔV_(LD) =α·ΔV_(sig).This change in applied voltage modulates the output light from thesemiconductor laser 70. When the output light from the semiconductorlaser 70 is measured, and the coupling efficiency α is calculated, anelectrical signal of the circuit 76 to be measured can be detected.

FIG. 11 is a circuit diagram when parallel plates are used as theelectrode 75 and the measurement point 76a of the circuit 76 to bemeasured. If the coupling area is represented by S, and the intervalbetween the electrode 75 and the measurement point 76a is represented byd, an electrostatic capacitance C between the electrode 75 and themeasurement point 76a is given by:

    C=εS/d

where ε is the dielectric constant. If the angular frequency of theinput signal source 73 is represented by ω, the impedance Z of theelectrostatic capacitance C is given by:

    Z=1/ωC

Since both the current and voltage of the semiconductor laser 70linearly change beyond the threshold current I_(th), the semiconductorlaser 70 can be replaced by an equivalent resistance R (=ΔV_(LD)/ΔI_(LD)). Therefore, V_(LD) is given by:

    V.sub.LD =R/(Z+R)·V.sub.sig

From this equation, we have:

    α=V.sub.LD /V.sub.sig =R/(Z+R)

Normally, since Z>>R, then:

    α=R/Z

The above-mentioned equations yield:

    α=εωSR/d

For example, if the input signal frequency f=100 kHz, the coupling areaS=1.0 mm², the interval d=0.1 mm, and the equivalent resistance R=5 Ω,we have: ##EQU1## For ε=8.85×10⁻¹² F/m.

FIG. 12 shows the distribution of actually measured data of a modulatedsignal voltage V_(mod) with respect to the amplitude voltage V_(sig) ofan electrical signal obtained when the electrode 75 is brought close tothe measurement point 76a in a non-contact manner while thesemiconductor laser 70 is CW-oscillating. Assume that a sine wave signalof a frequency of 50 kHz is applied to the circuit 76 to be measured.The modulated signal is detected by the lock-in amplifier 44. As can beseen from these actually measured data, the detected signal isproportional to the signal amplitude. As can be understood from theabove description, an electrical signal applied to the circuit 76 to bemeasured can be measured if the electrode 75 is not brought into contactwith the circuit 76 to be measured.

A modification of this embodiment will be described below with referenceto the perspective view in FIG. 13. In this modification, thelower-surface electrode 12a of the semiconductor laser 12 is used as avoltage detection electrode, thereby omitting the electrode 11. Outputlight from the semiconductor laser 12 is detected by the photodetector30 via the optical fiber 20 adhered to the output end face of thesemiconductor laser 12. Note that the semiconductor laser 12 and theoptical fiber 20 may be arranged to be separated by a predeterminedinterval in place of the direct connection. In this case, a focusinglens for focusing the output light from the semiconductor laser 12 ontothe optical fiber 20 is required.

FIG. 14 shows an arrangement of a probe unit 130 obtained when asurface-emission laser 131 is used in place of the semiconductor laser12. An upper-surface electrode 131b of the surface-emission layer 131 isdirectly adhered to a housing 130. As shown in FIG. 15, an incidentwindow 130a for receiving the output light from the surface-emissionlaser 131 is formed in the housing 130. Therefore, the housing 130 andthe surface-emission laser 131 are connected to each other, so that thelight output portion of the surface-emission laser 131 coincides withthe incident window 130a. As the surface-emission laser 131, a ridgetype one is used. For this reason, even when a bonding wire 132 is fixedto a lower-surface electrode 131a, it does not disturb measurements. Theoutput light from the surface-emission laser is focused by a focusinglens 133 via the incident window 130a, and is supplied to an opticalfiber 134.

A modification of the electrode 11 will be described below withreference to FIGS. 16 and 17. FIG. 16 is a perspective view showing amodification in which the electrode 11 is formed to have a dipoleantenna shape. The dipole antenna-shaped electrode 11 has high couplingefficiency when a measurement point 81a of a device 81 to be measuredhas a linear pattern. More specifically, when the electrode is broughtclose to the measurement point 81a in a non-contact manner with itsdistal end being directed to be parallel to the linear pattern of themeasurement point 81a, the coupling efficiency becomes very high. In acircuit with crossing linear patterns, the distal end of the electrodecan be aligned with a linear pattern to be measured, thus attaining ameasurement with less crosstalk from the other linear pattern.

FIG. 17 is a perspective view showing a modification wherein theelectrode 11 is formed to have a needle shape, and is covered with anon-conductive material 11a such as a ceramic. Since a high mechanicalstrength against bending can be assured by the non-conductive material11a, a thin electrode 11 can be used. For this reason, an electric fieldof a very narrow portion on the surface of a device to be measured canbe detected, and the spatial resolution can be improved.

FIG. 18 is a perspective view showing a modification in which asurface-emission semiconductor laser array 100 is used. FIG. 19 is asectional view showing the structure of this modification. Since thismodification adopts the semiconductor laser array 100, the surface of, adevice to be measured can be two-dimensionally measured. A plurality ofelectrodes 101 corresponding to semiconductor lasers are arranged on thelower surface of the semiconductor laser array 100. These electrodes 101are applied with a voltage from a power source. When the electrodes 101are brought close to an upper portion of a device 102 to be measured ina non-contact manner, the semiconductor lasers of the semiconductorlaser array 100 modulate their output light beams by an electricalsignal applied to the device 102 to be measured immediately therebelow.The modulated output light beams become incident on an optical fiberarray 104 via a SELFOC lens 103, and are then guided to a photodetector105. As the photodetector 105, for example, a photodiode array, CCDs,and the like are used. When detected signals from the photodetector 105are processed by a signal processor 106, electrical signals at aplurality of points can be simultaneously measured.

FIG. 20 is a perspective view showing an application in which thisembodiment is applied to an IC inspection apparatus. In thisapplication, an IC circuit board 110 is placed on an upper stage 112 ofa base plate 111, and the voltage applied to an IC circuit 110 formed onthe IC circuit board 110 is measured by a probe unit 114 fixed above thestage 112. The IC circuit board 110 and the probe unit 114 are alignedin the longitudinal and widthwise directions by moving the upper stage112 and a lower stage 113 in the right-and-left directions. The ICcircuit board 110 and the probe unit 114 are aligned in the verticaldirection by moving a movable stage 116 provided to a probe supportportion 115 in the vertical direction. The voltage applied to the ICcircuit 110 measured by the probe unit 114 is transmitted through anoptical fiber 117 as signal light. This signal light is detected by aphotodetector 118, and the detected signal is processed by a signalprocessor 119.

In this embodiment, the semiconductor laser 12 is used as a lightsource. However, the present invention is not limited to thesemiconductor laser. For example, a light-emitting diode may be used.

The optical fiber 20 is not an indispensable component of the presentinvention. For example, the photodetector 30 may be arranged on theoptical axis of the focusing lens 13, and the probe unit 10 and thephotodetector 30 may be integrated.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A voltage detection apparatus for detecting avoltage applied to a local portion of a device to be measured,comprising:a detector for detecting a change of a strength of anelectric field in a space between said detector and said device to bemeasured and generating a signal component corresponding to the changeof the strength of said electric field; a constant current source forsupplying a bias current; means for superposing said signal componentfrom said detector on said bias current from said constant currentsource; light-emitting means for modulating output light, based on asupplied current from said means; and extraction means for extractingsaid signal component of said output light from said light-emittingmeans.
 2. An apparatus according to claim 1, further comprisinglight-transmission means for guiding said output light from saidlight-emitting means to said extraction means.
 3. An apparatus accordingto claim 2, wherein said light-transmission means comprises an opticalfiber.
 4. An apparatus according to claim 1 further comprising:opticalmeans for focusing said output light from said light-emitting means toan input terminal of said light-transmission means; and a housing forstoring said light-emitting means and said optical means, wherein saiddetector means is fixed to said housing, and said light-transmissionmeans is inserted in said housing, so that said input terminal isarranged on an optical axis of said output light from saidlight-emitting means.
 5. An apparatus according to claim 1 wherein saidlight-emitting means comprises a semiconductor laser.
 6. An apparatusaccording to claim 5, wherein said detector comprises a rod-shaped metalelectrode with a sharp distal end, and wherein said detector and anupper electrode of said semiconductor laser are connected to a firstelectrode of said constant current source, and a lower electrode of saidsemiconductor laser is connected to a second electrode of said constantcurrent source.
 7. A voltage detection apparatus for detecting a voltageapplied to a local portion of a device to be measured comprising:ahousing; a detector supported by said housing and having a portionprojecting to an exterior of said housing, for detecting a change of astrength of an electric field in a space between said detector and saiddevice to be measured and generating a signal component corresponding tothe change of the strength of said electric field; a constant currentsource for supplying a bias current; means for superposing said signalcomponent from said detector on said bias current from said constantcurrent source; light-emitting means provided in said housing, formodulating output light, based on a supplied current from said means;and extraction means for extracting said signal component of said outputlight from said light-emitting means.
 8. An apparatus according to claim7, further comprising light-transmission means for guiding said outputlight from said light-emitting means to said extraction means.
 9. Anapparatus according to claim 8, wherein said light-transmission meanscomprises an optical fiber.
 10. An apparatus according to claim 7,further comprising:optical means for focusing said output light fromsaid light-emitting means to an input terminal of saidlight-transmission means, wherein said detector is fixed to saidhousing, and said light-transmission means is inserted in said housing,so that said input terminal is arranged on an optical axis of saidoutput light from said light-emitting means.
 11. An apparatus accordingto claim 7, wherein said light-emitting means comprises a semiconductorlaser.
 12. An apparatus according to claim 11, wherein said detectorcomprises a rod-shaped metal electrode with a sharp distal end, andwherein said detector and an upper electrode of said semiconductor laserare connected to a first electrode of said constant current source, anda lower electrode of said semiconductor laser is connected to a secondelectrode of said constant current source.